Serial Attached SCSI technologies and architectures, 4th edition

Expanders

Expanders connect initiators, targets, and other expanders. They receive commands and data in one

port and route them to another port based on the SAS address of the target. Expanders use three

routing methods: direct, table, and subtractive. Direct routing forwards the commands and data to

targets directly attached to the expander. Table routing forwards the commands and data to another

expander. When an expander receives a SAS address that it does not recognize, it uses subtractive

routing to forward the commands and data to another expander that recognizes the address. Each

routing method uses routing tables that are maintained in each expander. The expander creates the

routing table during the discovery process known as self-configuration.

Targets

SAS drives (both enterprise-class and midline) have two narrow ports. SAS drives use the same

electrical and physical connection interface as SATA drives. However, SATA drives have a single

narrow port. You can have SAS and SATA devices in a single domain. The size of the expanders’

routing tables determines how many initiators and targets you can have in a domain.

SAS protocol

SAS uses a point-to-point architecture that transfers data to and from SCSI storage devices by using

serial communication. SAS devices use differential signaling to achieve reliable, high-speed serial

communication. SAS inherits its command set from parallel SCSI and its frame formats and full-duplex

communication from Fibre Channel. SAS also supports SATA targets.

Second-generation SAS (SAS-2) doubles the physical link rate to 6.0 Gb/s. SAS-2 added self-

configuring expanders. SAS-2 includes zoning capabilities to improve resource deployment flexibility,

security, and data traffic management. SAS-2 maintains backward compatibility with SAS-1.

SAS-2 devices (initiators, targets, or expanders) can support more than one communication speed. If

any two linked devices support multiple speeds, the devices use the highest mutually supportable

speed. The linked devices determine that speed during a speed negotiation process at start up. A

sequential series of speed negotiation windows (SNW) characterizes this process. In SNW-1 and

SNW-2, linked devices test established combinations of SAS speed(s), transmission amplitude, slew

rate, de-emphasis, and spread spectrum clocking (SSC). In SNW-3, the linked devices negotiate link

speed and SSC settings.

Unlike SAS-1, SAS-2 allows for training of the transceiver mechanism (PHY) and for exchanging

parameters. After SNW-3 has negotiated the speed and settings, a training-speed negotiation

window (Train-SNW) tests the fastest mutually supported speed.

The SAS-2.1 standard defines active cables, storage power management, and additional connectors.

Also, SAS-2.1 splits out the protocol layer into a separate standard, SAS Protocol Layer (SPL).

Active cables

The SAS-2.1 standard supports active cables, which are thin cables with active circuitry to reduce

cable weight, improve cable management, and improve airflow. Active circuitry includes built-in

drivers, repeaters, and an equalizing filter. The equalizer removes inter-symbol interference (ISI), a

form of signal distortion. The drivers and repeaters increase the signal-to-noise ratio by boosting the

received signal and reducing near-end crosstalk (NEXT). NEXT occurs when two wires are close

enough for the signal traveling in one wire to interfere with the signal traveling in the other. Active

cables include a low-power equalizing filter to compensate for the dielectric and conductor losses and